The effects of the molecular weights (molecular weight of polystyrene, M w,PS , varying from 2.9 to 129k) on the surface morphologies of spin-coated and annealed polystyrene/poly (methyl methacrylate) (PS/PMMA=50/50, w/w) blend films were investigated by atomic force microscopy and X-ray photoelectron spectroscopy. For the spin-coated films, when the M w,PS varied from 2.9 to 129k, three different kinds of surface morphologies (a nanophase-separated morphology, a PMMA cellular or network-like morphology whose meshes filled with PS, a sea-island like morphology) were observed and their formation mechanisms are discussed, respectively. Upon annealing, two different morphology-evolution processes were observed. It is found that a upper PS-rich phase layer is formed when Mw,PS<4k, and this behavior is mainly attributed to the low interfacial tension between PS and PMMA component. When Mw,PS>4k, the PS-rich phase forms droplets on top of the PMMA-rich phase layer which wets the SiO x substrate. These results indicate that the surface morphology of the polymer blend films can be controlled by the polymer molecular weight and annealing conditions.